1. Field of the Invention
This invention relates to a transfer medium separating device for separating a transfer medium bearing thereon a toner image from a carrier member with which the transfer medium is in contact. More in particular, the present invention is concerned with a copy paper separating device in an electrophotographic copying machine for separating a copy paper from the surface of a photosensitive member.
2. Description of the Prior Art
In a transfer type electrophotographic copying machine, a photosensitive member, usually provided on the peripheral surface of the drum which is driven to rotate at constant speed, is first uniformly charged to a predetermined polarity by a corona charger. Then, a light image is exposed onto the thus charged surface of the photosensitive member thereby selectively removing the charges to form an electrostatic latent image by the remaining charges. Torner particles charged in the polarity opposite to that of the electrostatic latent image are applied to form a visible toner image. Next, a transfer medium such as a copy paper is brought into contact with the surface of the photosensitive member bearing thereon the toner image.
At this transferring station, a corona charger is usually used to apply charges of the polarity opposite to that of the toner image onto the backside of the copy paper so that the toner image may be electrostatically attracted to the front side of the copy paper. Thereafter, the copy paper, which is now in contact with the surface of the photosensitive member at least partly, is separated from the surface of the photosensitive member and then transported to a fixing station where the transferred toner image is permanently fixed to the copy paper by fusing. On the other hand, the photosensitive member is then subjected to a cleaning operation to remove any residual toner particles, and thereafter the photosensitive member is prepared for the next cycle of copying operation.
This type of electrophotographic copying machine has become very popular and is widely used partly because of its ability to use a plain copy paper. Among the many factors considered in judging the performance of this type of copying machine, the transferring efficiency, i.e., the ability to transfer the toner image from the photosensitive member to the copy paper, is often one of the most important factors. In order to obtain high transferring efficiencies, it is a common practice to bring the copy paper in intimate contact with the photosensitive member. This then leads to a problem of separating the copy paper from the photosensitive member since they are strongly attracted to each other.
One conventional and common approach was to use a separating pawl in combination with a charge neutralizer which is comprised essentially of a corona charger. That is, in accordance with this prior art technique, after the application of the transferring charges, the copy paper is electrostatically neutralized and the leading edge of the copy paper is brought into engagement with the pawl as the photosensitive drum rotates and the copy paper is thereby mechanically separated from the photosensitive member gradually from its leading edge. However, there are numerous disadvantages in this prior art technique.
One of the disadvantages of the separating pawl approach is the difficulty for appropriate neutralization of the copy paper. For example, if the neutralization is insufficient, the copy paper may strongly adhere to the photosensitive member, which, in turn, could cause problems such as tearing of the copy paper during the separating operation. On the other hand, if an excessive neutralization is carried out, the electric field for attracting the toner particles to the copy paper becomes weaker, which, in turn, could bring about lowering of the transferring efficiency as well as the image density. Another disadvantage stems from the fact that the pawl is provided to be always in contact with the surface of the photosensitive member. This structure easily allows scratches to be formed on the photosensitive surface, for example, when a foreign matter such as debris gets sandwiched between the pawl and the surface. This is especially true for a photosensitive member comprised of organic materials.
Another proposed approach was to place a conductive member in the neighborhood of the copy paper which has been subjected to the transfer operation. In this case, an electrostatic attractive force is induced between the copy paper and the conductive member so that the copy paper may be removed from the photosensitive member. However, no practically applicable structure has yet been proposed.